Another aha! moment.
Rock object.....scale 80,305,40, rotate 0,60,180 (Because this sits way off top of volcano)
Shattering 0, 10, 0.28
Mesh modifiers, subdivide 2
Default shader Diffuse 0.94
I have not tried translucency, luminosity, opacity yet.....but this is a BIG starting place.
You want a narrow z with a fat x base with a tall y rock.